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TOPIC: Mercruiser "Guardian Mode" question

Mercruiser "Guardian Mode" question 19 Sep 2017 22:44 #1

  • iceman11
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I posted this in an older thread but it might be worthy of its own thread. I have an engine problem where I can't get enough power to get on plane; basically the RPM tops out at ~2500 no matter how high I push the throttle. There are no fault codes or alarms so I've been tearing my hair out trying to find the cause.

The Mercruiser MPI engine has "Guardian Mode" where the computer limits the engine RPM if it thinks there is a fault that could cause engine damage. Normally (maybe always?) this happens when the engine alarm is heard. Is it possible that the engine is being put in Guardian mode in error, with no fault being sensed or by some condition that doesn't generate an alarm or fault code? Or that the computer itself has a fault that is forcing Guardian mode. From all I've read I believe that this SHOULDN'T happen, but the question is, is it POSSIBLE? If so, how would I be able to diagnose that?

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2006 Maxum 2600SE with Merc 5.0L MPI/Bravo III

Mercruiser "Guardian Mode" question 19 Sep 2017 23:27 #2

  • Nickdixon
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I know one condition that does NOT give an alarm and that is low temp, but only after running for a few minutes. Evidently, by the time the engine should close the thermostat, my 4.3 behaves just like you describe. Thermostat wide open due to debris.

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Bayliner 195 Bowrider 2013 4.3l 220hp MPI
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Mercruiser "Guardian Mode" question 19 Sep 2017 23:41 #3

  • watchthis
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My guardian does/did not make any alarm sound. About a year ago I was cruising at 3600 rpms hitting around 20 knots when the system abruptly went into safe mode as the stainless steel shaft in the water pump had broken cleanly in two (manufacturing flaw) causing an abrupt temperature spike. Pretty strange sensation for a gauge watcher like me to get knocked down off plane so quickly without an audible alarm, but thank god it did as it saved my engine. I wish knew if there was a scanner you could buy or rent to sort through a code, but I just don't know.

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Johnson Point, Olympia, WA
1989 2855
Horizon 6.2 and Bravo II

Mercruiser "Guardian Mode" question 19 Sep 2017 23:43 #4

  • watchthis
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Also, no code was evident on the display, just the elevated temperature. I got to spend the next day figuring out what had happened.

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Johnson Point, Olympia, WA
1989 2855
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Mercruiser "Guardian Mode" question 20 Sep 2017 04:20 #5

  • Redwngr
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iceman11 wrote: I posted this in an older thread but it might be worthy of its own thread. I have an engine problem where I can't get enough power to get on plane; basically the RPM tops out at ~2500 no matter how high I push the throttle. There are no fault codes or alarms so I've been tearing my hair out trying to find the cause.

The Mercruiser MPI engine has "Guardian Mode" where the computer limits the engine RPM if it thinks there is a fault that could cause engine damage. Normally (maybe always?) this happens when the engine alarm is heard. Is it possible that the engine is being put in Guardian mode in error, with no fault being sensed or by some condition that doesn't generate an alarm or fault code? Or that the computer itself has a fault that is forcing Guardian mode. From all I've read I believe that this SHOULDN'T happen, but the question is, is it POSSIBLE? If so, how would I be able to diagnose that?


On mine, the 'guardian' will give a series of beeps and then after perform as you are describing, limiting the power level/rpm.

It only gives the series beeps once, and will run indefinitely with the restricted power level.

The manual indicates that there are different levels of power restriction that can be applied, depending on the nature and seriousness of the detected fault.

(( 6 beeps (one sec on, one sec off) for one level of fault, one 6 second beep for another level ))

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Mercruiser "Guardian Mode" question 23 Sep 2017 17:04 #6

  • Ruffryder
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Ice, is your alarm even working at the startup?
not normal in both you & watchthis's case.
It would be nice to get a hand on a spare ECM to be swapped...
Is the battery fully charged and in good condition?

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Mercruiser "Guardian Mode" question 23 Sep 2017 21:00 #7

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Nickdixon wrote: I know one condition that does NOT give an alarm and that is low temp, but only after running for a few minutes. Evidently, by the time the engine should close the thermostat, my 4.3 behaves just like you describe. Thermostat wide open due to debris.


I'm reposting this because I also need to know if the guardian mode always sounds an alarm when activated. In my case, prolonged low temp, I have not heard an alarm but I definitely am limited in rpm. So, the Mercruiser owners manual for my 4.3 MPI, says "the system will respond to a problem by emitting a continuous beep and/or reducing engine power..." That language leaves open the the no alarm but reduction in power situation, contrary to what seems to be the popular, and perhaps correct opinion on BOC.

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Bayliner 195 Bowrider 2013 4.3l 220hp MPI
Alpha 1, Gen II
2012 Toyota Highlander V6 4wd with tow package
Albany, Ohio

MMSI: 338234042


Beware the men of measured merriment
(Sinclair Lewis, Arrowsmith, paraphrased)

Mercruiser "Guardian Mode" question 24 Sep 2017 18:45 #8

  • iceman11
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Ruffryder: yes the alarm sounds at startup, so I know that part is working. Batteries are good.

Where I stand on this is, 5 of the valve seats (4 exhaust, 1 intake) have been machined and it seems that they were leaking. Prior to this, we saw a steady low manifold vacuum, where normally we'd expect it to be fluctuating if a valve leaks, so it's a little confusing. The engine is being put back together, and sometime in the next few days I'm going to launch to test this.

If the power loss remains I may be back to checking for Guardian mode. One thing I should mention: the engine telemetry reports that the available power is 100%. Does this mean it can't be in Guardian mode? If it was in Guardian mode would it report < 100% available power?

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2006 Maxum 2600SE with Merc 5.0L MPI/Bravo III

Mercruiser "Guardian Mode" question 24 Sep 2017 20:05 #9

  • watchthis
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I have owned my boat for a little of over a year. Never any alarm at start up. All monitoring seems to be accurate, batteries are in top notch condition.

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Johnson Point, Olympia, WA
1989 2855
Horizon 6.2 and Bravo II

Mercruiser "Guardian Mode" question 26 Sep 2017 00:27 #10

  • captharv
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I recently had an issue with mine. When I was cruising, it was fine. Slowed for a no-wake zone or an anchored boat in the channel, the RPMS were limited to 2500. Would not plane. After maybe 2-3 minutes, it would suddenly return to normal and run right.
When I got home, I read up on it. The guardian mode is there for when the engine runs too hot, and limits RPMs in an attempt to let the engine cool better. The sensor in mine is separate from the gauge sender. The sender goes to the computer. (Thunderbolt V in my case) so I measured the sender with my DVM. It read open. I wacked the sender housing with a wood stick and it read about 100 ohms. I went to the merc dealer and a new one read 100 ohms and did not change while tapping it on the counter top. I bought it and installed it, no more problems. (The parts manager said they sell a lot of them)
That said, check the wiring. The sender has a small connector which is allegedly waterproof (yeah right). look for corrosion on the contacts and wires attached. Check the connector on the computer/ICM/ whatever its called. You see, the wiring can be bad at either end.
I am not sure if the oil and outdrive senders are also wired in on the MPI engines. READ THE MANUAL.

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Captharv 2001 2452
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